Biological implants such as artificial bones or tooth roots have recently attracted attention because when bones or teeth are broken or otherwise damaged or destroyed by an accident, etc., the implants can be bonded to the remaining bone or implanted, e.g., in the case of tooth roots, in the bones of the jaw, and thus can be used in a form close to natural bones or teeth and ensure maintenance of comfortable daily lives. Since these implants are to be embedded in the body, they should be nontoxic to the body. They also must have various other properties, such as sufficient strength, good moldability or workability, freedom from dissolution, moderate specific gravity, and biocompatibility.
Alpha-alumina, noble metals and alloys such as stainless steel have been used as implants but they fail to satisfy one or more of the requirements set forth above. Furthermore, they have a common problem in that they are not biocompatibile.
Recently, apatite ceramics have been proposed as materials that are claimed to satisfy all of the requirements that should be met by ideal implants. Apatite ceramics are chiefly composed of calcium phosphate compounds. Since the main components of bones and teeth are also calcium phosphate compounds, apatite ceramics exhibit very good biocompatibility and can be readily assimilated by living tissues after they are implanted in the body. However, apatite ceramics still suffer the problems of low strength and low workability and their use has been limited.
In order to eliminate these problems, techniques are required that are capable of joining ceramics to either metals or ceramics to produce strong adhesion. Plasma spraying and sputtering are two known techniques suitable for this purpose. While the plasma spray method is useful in achieving the joints described above, it has various defects: for example, difficulty in applying uniform coatings on the entire surface of a complexly shaped material exists; all surfaces of a porous material cannot be coated; an expensive apparatus is required; the efficiency of utilization of expensive apatite particles is low; and strong adhesion between the coating and the substrate is not always provided. A sputtering process also has several defects, such as high cost and the inability to produce coatings thicker than 1 .mu.m and, therefore, the thickness of coatings cannot be appropriately adjusted in accordance with the specific use.
With a view to eliminating the defects of apatite ceramics, the present inventors previously proposed a process for forming a layer of calcium phosphate compound on a substrate by firing a coating made from a nitric or hydrochloric acid aqueous solution of a calcium phosphate compound, as well as a composite material produced by this process (see Japanese Patent Application No. 64012/86). The proposed method has the advantage that a uniform coating layer can be formed on the surface of a substrate of any shape. However, in this method, the calcium phosphate compound is directly precipitated from its aqueous solution and deposited on the surface of the substrate, so the adhesion between the substrate and the deposited coating of calcium phosphate compound is rather weak and prone to failure (i.e., delamination) on extended use.
As will be described immediately below, a principal object of the present invention is to attain strong adhesion between a substrate and a calcium phosphate compound by employing an organic compound. The use of an organic compound in the production of hydroxyapatite is known and in the process described in JP-A-61-295215 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), hydroxyapatite is produced through the reaction between a calcium alkoxide and phosphoric acid. But this method is directed to the production f a hydroxyapatite powder itself and is not intended for improving the adhesion between a coating of this hydroxyapatite and a substrate.